This invention relates generally to a sound absorber with multiple layers of sound absorbing material and a method of manufacturing a sound absorber, and in particular, to a method of directly filling a sound absorber with different types of sound absorbing material. The invention is useful in the production of sound absorbers that may be used to reduce noise emissions of a vehicle.
Sound absorbers are typically used to reduce noise emissions and have numerous applications, for example, a muffler for a vehicle. A conventional sound absorber includes a housing or container, usually cylindrical or oval cross-section, with a perforated or porous inner tube extending through the end pieces of the container through which a gas, such as exhaust gas from an internal combustion engine, can flow. The sound absorber often includes a sound absorbing material, such as fiberglass wool, that is disposed between the housing and the inner tube and that dampens or attenuates noise in the gas flowing through the muffler.
Such mufflers may be manufactured in several ways. The fiberglass wool may be pressed in the form of a mat between the housing and the inner tube or rolled around the inner tube. U.S. Pat. No. 5,926,954 to Wolf (xe2x80x9cWolfxe2x80x9d) relates to a method of making a multiple fiber layer muffler. The layers of fiber yam are wrapped around an inner perforated tube as the tube rotates. A septum is placed between the layers to separate them from each other. The multiple layers and septums reduce blowout of the fibers (which decreases the acoustic reducing properties of the muffler). A drawback of the method of Wolf is that it requires the sound absorbing material to be in the form of a mat, thereby limiting its acoustic and thermal performance. The nature of the winding process results in parallel filaments that limit the thermal insulation properties of the fibrous material. Because of the required tension of the winding process, such fibers tend to be tightly packed against the perforated tube and equilibrate to the high temperatures of the exhaust gas resulting in a greater susceptibility to blowout. Also, the method is time consuming and expensive, and it requires multiple pieces of machinery.
Alternatively, the fiberglass wool may be in the form of expanded, chopped fiberglass with a fiber length of approximately 50 mm. Using such chopped strand fiberglass requires expensive equipment for filling the mufflers and makes it difficult to fill the muffler evenly. Both the chopping and needling processes impart severe damage to the fibers with a loss of more than 50% of the fibers"" tensile strength. Additionally, such a construction exhibits poor durability since a majority of the chopped fibers are often less than 15 mm in length. These very short fibers eventually migrate through the muffler perforations and blow out. Uneven filling can also result in the wool being packed against the cylindrical inner wall of the housing by the exhaust gases passing through the inner tube, which in turn leads to the noise reducing performance of the muffler deteriorating relatively quickly. This process has high fabrication costs due to the amount of labor, number of preparation steps, waste, and difficulties in filling complex designs.
One design consideration for sound absorbers is the thermal degradation of the sound absorbing materials over time from exposure to hot exhaust gases. One design approach involves the use of more temperature resistant materials near the inner tube of the muffler, where the temperatures are higher. U.S. Pat. No. 4,269,800 to Sommer et al. (xe2x80x9cSommerxe2x80x9d) discloses muffler designs that use both individual mats of mineral fibers and metal fibers and a composite mat consisting of both types of fibers. Sommer discloses a muffler with separate layers of mineral and metal fibers. The metal fiber is positioned closer to the combustion gas in the muffler than the mineral layer to provide heat and corrosion resistance protection. The two layers are needled or sewn together, or coupled with an adhesive. Sommer also discloses a method of manufacturing a composite mat by combining metal fibers with mineral fibers during the manufacture of a mineral fiber mat. It is time consuming and labor intensive to manufacture and insert the mat. Furthermore, the Sommer design offers only an incremental improvement in durability versus standard glass fiber mats. The design consists of discontinuous fibers which, under vibrational loading in the exhaust, will eventually migrate through the perforations and blow out of the muffler.
A need exists for an inexpensive way to manufacture a sound absorber that includes different types of sound absorbing fibers. The absorber would preferably contain continuous fibers which will not be predisposed to blow out of the muffler through perforations in the inner tube. By using different types of fibers, more expensive, temperature resistant fibers may be placed closer to the exhaust gas to protect the inexpensive fibers that are less resistant to heat.
The shortcomings of the prior art are overcome by the disclosed multiple layer fiber filled sound absorber and method of manufacturing the absorber. The sound absorber includes an outer housing, a porous or perforated inner tube or housing defining a passageway through which a gas may flow, and two layers of sound absorbing material, such as fiberglass wool, is positioned between the housings. The sound absorbing material adjacent the inner housing is selected to be more heat resistant than the material farther away from the inner housing.
The sound absorber is filled with the sound absorbing materials using a direct fill process in which continuous fibers are injected into the sound absorber. In preparation for filling, a partition is positioned between the housings to define two chambers. Each chamber is filled with one of the sound absorbing materials. The partition may then be removed from the sound absorber, or may be left in place (in which case the partition is preferably porous or perforated). The two sound absorbing materials may be filled to different densities. If the partition is removed, the densities will tend to equalize.
The direct fill process simplifies and reduces the cost of filling the container and provides a muffler that is uniformly filled and has an improved sound absorbing quality.